Why does auto-suggest not work for all users?

I have run into kind of a weird situation with OnDemand. For all but one of my team users, this feature works, but for him, it does not. I've eliminated any Permissions issues, etc, but this one thing is still annoying me.

1. User goes to Add Watcher by clicking on the number next to the 'Watching' link.

2. Add Watcher dialog box comes up.

3. User starts typing name...but no other usernames are displayed.

For everyone else on the team, as soon as the first letter is typed, we get a list of any team members with that letter as the first letter in their name, e.g. "A for Alex"...just not this one user. He's never shown that list of auto-suggested names. If he enters the users email address, he can add a watcher, but that's the ONLY way.

There's no difference in the browser type (Chrome), and I can't figure out whatever else it could be.